This section introduces aspects that may be helpful in facilitating a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Every few years, wireless cellular communications got renewed. While the move to 2nd Generation (2G) has been technology driven, i.e., the switch to digital signal processing, the switch to 3G (data, internet access) and 4G (video) have been service driven to a high extend. The achievable data rates have grown significantly starting from few kbit/s (beginnings of the Global System for Mobile Communications, GSM) up to a few hundreds of kbit/s (Enhanced Data rates for GSM Evolution, EDGE) for 2G. The beginnings of 3G have reached several hundreds of kbit/s increasing up to 42 Mbit/s (at least theoretically) with High-Speed Downlink Packet Access (HSDPA) the newest incarnation of 3G. Actually 4G is rolled out achieving up to a few hundreds of Mbit/s (Long-Term Evolution, LTE) using multicarrier techniques, with its evolution LTE advanced at the horizon approaching the Gbit/s region.
Multicarrier signal formats offer a large amount of flexibility. This flexibility is very attractive when aiming at a scalable radio frame structure. There is also a natural, physical explanation why multicarrier signal formats are desirable. Wireless propagation channels are linear and—approximately over the duration of one multicarrier symbol—time-invariant (LTI). LTI systems preserve sinusoids as eigenfunctions. Sinusoids are one basic building block of multicarrier modulated signals. This leads to nice properties when it comes to demodulation and equalization.
One significant drawback of multicarrier modulation is the Peak-to-Average Power Ratio (PAPR), but there are methods like Discrete Fourier Transform (DFT) precoding used in conjunction with Orthogonal Frequency Division Multiplexing (OFDM) to build up Single-Carrier Frequency Division Multiplexing (SC-FDMA), out of multicarrier signals, which brings down PAPR.
OFDM is today's dominant multicarrier technology. Its Cyclic Prefix (CP) allows transforming the linear convolution of the channel into a cyclic convolution, thus deals very elegantly with multi-path propagation, at the price of additional overhead of the CP, typically ranging from 5-25%. As long OFDM is used in a fully time- and frequency synchronous manner this is very attractive.
5G systems will bring along new device classes and new traffic types, e.g., driven from the Internet of Things (IoT). Relaxing synchronicity will allow for reducing a painful overhead for massive numbers of machines. Technologies like Autonomous Timing Advance (ATA) can be used. ATA here means an open-loop timing control approach, where a mobile terminal synchronizes itself on a downlink receive signal, e.g. using pilot symbols and/or synchronization sequences and corrects its timing autonomously, e.g. based on knowledge of supported cell sizes, etc. Additionally, low-end devices can be made cheaper when e.g. oscillator requirements can be relaxed, which are very strict for e.g. LTE. On top, the trend towards higher carrier frequencies, like millimeter waves, causes that the same relative Carrier Frequency Oscillator (CFO) requirements will lead to much larger absolute frequency shifts, phase jitters etc. observed in the baseband processing.
The demand for increased robustness and for relaxation of strict time- and frequency alignments does not go together well with OFDM. In the OFDMA uplink, when devices allocated to neighboring frequencies have timing- and frequency offsets, orthogonality is lost and Inter-Carrier Interference (ICI) is generated, reducing the overall system performance. Due to the rectangular-windowed time domain shape of OFDM symbols, the subcarrier spectrum is formed of sinc-functions which have comparatively high side lobe levels. Only with strict time- and frequency alignments, OFDM can be attractive when the nulls of the spectral subcarrier levels fall together with the maxima of other subcarriers.
Existing multicarrier alternatives to OFDM aim at reduced spectral side-lobe levels, making them more attractive for e.g. uplink FDMA with asynchronous users and operation in fragmented spectrum. Filter-Bank based Multi-Carrier (FBMC) is using additional per-subcarrier pulse-shaping filters, typically with a length of more than one multi-carrier symbol. Those filters provide very strong side-lobe suppression and can be implemented efficiently in poly-phase filter-banks. The CP overhead can be avoided. Due to the long filter lengths, FBMC is best used in conjunction with offset-QAM (OQAM) with subsequent symbols overlapping, but being orthogonalized by using real and imaginary part of the symbol alternately. Drawbacks of FBMC are that—due to OQAM—it is e.g. not compatible to all kinds of Multiple Input Multiple Output (MIMO). Furthermore, the long filter lengths make short burst inefficient, due to filter “ramp up” and “ramp down” time. Short bursts will be important for energy-efficient Machine Type Communication (MTC). Here, another existing multicarrier signal format hooks in: Generalized Frequency Division Multiplexing (GFDM). It is similar to FBMC with subcarrier-wise pulse shaping, but uses methods like tail biting (circular convolution with the filters instead of linear convolution) to be attractive for short bursts. Its drawbacks are overlapping subcarriers and thus usually comparatively complex receivers. Filtered OFDM, thus filtering OFDM over the entire band, is known and used for some time to reduce out-of band radiation.
It is desirable to provide more multicarrier alternatives to OFDM.